For the study of the regulation of mucin synthesis and secretion hamster tracheal epithelial cell cultures grown on collagen gels appear to provide us with a good model system. Cells were radiolabeled with 3H-glucosamine and/or 35S-sulfate and their secretory products were characterized. Mucins were identified on the basis of their high molecular weight and complete resistance to proteoglycan-degrading enzymes, ruling out proteoglycans. Labeling with several monosaccharide precursors and analysis of the high molecular weight products by strong acid hydrolysis or neuraminidase treatment showed the presence of sialic acid but not mannose. Beta-Elimination released oligosaccharides with the conversion of most of the N-acetylgalactosamine into N-acetylgalactosaminitol, indicating that the oligosaccharides are O-glycosidically linked with protein. This mucin has been further characterized via Beta-endogalactosidase treatment and it was shown that R-G1cNacBeta1-3Ga1Beta1-R linkages are present. This finding is supported by lectin affinity chromatography on DSA-agarose gels. Studies on the action of retinoids and monensin on mucin secretion are in progress. For the study of the regulation of differentiation rabbit tracheal epithelial cell cultures are used as a model system. These cells undergo terminal differentiation into squamous, cornifying cells when reaching confluency. This process is inhibited by retinoids and promoted by calcium and serum. Several biochemical changes accompany this change in phenotype. Undifferentiated cells produce large amounts of hyaluronic acid whereas in keratinizing cells the synthesis of hyaluronic acid is very much reduced. Cells undergo quantitative as well as qualitative changes in keratin proteins as shown by 2-D electrophoresis and by immunoblot analysis using monoclonal antibodies. A 48 kd keratin appears to be a good marker for this differentiation. Differentiation into mucin secretory cells can be induced when cells are grown on collagen gels in the presence of retinoic acid. Retinoic acid appears to have a key role in the control of differentiation.